There are various structures based on phase-locked loops (PLL) and direct synthesis that can be used in a base station environment to realize digital channel selection, synchronization and narrow-band filtering.
FIG. 1 is a schematic block diagram of a simple integer-N PPL synthesizer structure comprising error detector 11, loop filter 12, VCO 13 and divider 14 blocks. The VCO block 13 has an input 15a and an output 15b. 
The VCO output 15b is divided by N in the divider block 14 and is fed back to the error detector block 11, which comprises a phase detector 16 and a charge pump 17. The error detector block has two-phase detection input ports, one for the feedback signal 18 from the VCO block and one for a fixed external reference signal 19 from a crystal oscillator. The error detector block compares the signals that are input.
The error signal is given bye(s)=Fref−FO/N, where e(s) is error signal, Fref the fixed external reference signal, and FO/N is the VCO output signal divided by an integer N.
When e(s)=0, i.e. when the two signal input to the error detector block are equal in phase and frequency, the error will be zero and the loop is said to be in a locked condition, andFO=N*Fref 
When e(s)≠0, i.e. when FO≠N*Fref, the error detector block will drive the VCO.
The VCO is a voltage driven device and the charge pump of the error detector block is operating as a current source/sink device. The charge pump, the loop filter and the VCO form an integrator, which changes its output frequency in a direction of reducing the error signal e(s).
The VCO signal output frequency will be changed by KV*ΔV, where KV is the VCO sensitivity (in MHz/Volt) and ΔV is the change in VCO input voltage. The frequency adjustment procedure will be repeated until the error signal e(s) is zero, and the loop is locked.
FIG. 2 is a schematic circuit diagram of the VCO block 13 as being comprised in the synthesizer of FIG. 1. The VCO block 13 comprises an oscillator 21 and a mechanism 22 for frequency tuning, which may be achieved by tuning of a resonance frequency.
The oscillator comprises typically an active device, such as a bipolar transistor 23, and a resonator structure 24. For a LC network based resonator, the resonance occur at f0=1/(2π(LC)1/2). This means that for a fix inductance, the resonance frequency is tuned by tuning the capacitance. The mechanism for frequency tuning is thus typically realized by a varactor, such as a voltage controlled tunable capacitor or a FET voltage dependent capacitor.
The resonator structure is typically realized as an essentially L shaped microstrip inductor line connected to one or several capacitors. The circuit is formed on top of a ceramic substrate having a high dielectric constant.